Piezoelectric film has been used to make many different types of sensors. One type of sound transducer that can be made using this technology is a microphone. Such a microphone is frequently constructed in the prior art by stretching a film membrane tight between two or more attachment points, allowing the film membrane to serve as a moving diaphragm. Sound causes the film diaphragm to vibrate. The vibration of the film generates an electric voltage across the two surfaces of the film which is then amplified and fed into a communication system. One significant limitation of this type of microphone is that it cannot operate in harsh environments where water or water vapor is present. Placing a waterproof membrane over the face of the vibrating diaphragm drastically reduces and almost eliminates the sound reaching the diaphragm and resulting in vibration of the diaphragm.
Conventional use of piezoelectric film in microphones involves the film being stretched between points or across a ring to provide stress in the film. When sound strikes the film, the film vibrates. This mechanical vibratory motion is what causes the film to produce an oscillating voltage field between the two sides of the film. If the film is not stretched tight, it is less sensitive to sound pressure waves, and therefore the oscillating voltage field between the two sides of the film is significantly reduced. If the volume of the sound source is increased, or the sound source is moved closer to the piezoelectric film, the sound level striking the film is greater and therefore the signal emitted from the film is increased.